import java.util.Scanner;

public class Main1329 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b =sc.nextInt();
        int c=sc.nextInt();
        double S= ((a + b) * c )/ 2.0;
        System.out.println(String.format("%.1f",S));
    }
}
